1. Field of the Invention
The present invention is related to the field of reflective liquid crystal display panels, and more specifically to additional structure within such panels for improving the available contrast ratio.
2. Description of the Related Art
Reflective panel LCD projectors use reflective liquid crystal display panels. These panels are illuminated by background light, and modulate spatially the amount of reflected light.
Referring to FIG. 1, a prior reflective liquid crystal display panel 30 is described. The panel 30 is illuminated by background light incident upon it from the normal direction 34. The panel includes a transparent cover plate 42, onto which are attached transparent electrodes 44. The device has a substrate 50, onto which the reflective pixels are defined. A space between the transparent cover plate 42 and the substrate 50 is filled with liquid crystal material 70.
The reflective pixels are made by reflective pixel electrodes 52, 54, 56, 58, formed on respective pixel control transistors 62, 64, 66 and 68. The reflective pixel electrodes 52, 54, 56 and 58 apply electric fields to respective adjacent portions of the liquid crystal material 70. This controls optical properties of the adjacent portion of the liquid crystal material 70, which in turn controls the amount of light that reaches each reflective pixel electrode, and returns back out through the liquid crystal material 70.
A problem in the prior art arises from reflections from the substrate 50 in the inactive areas 82, 84, 86. These inactive areas 82, 84, 86 are between the reflective pixel electrodes 52, 54, 56 and 58, and are also known as border spaces. Reflections from these border spaces 82, 84, 86 limit the amount of optical contrast available from the panel.
Referring now to FIG. 2, the prior art problem is described in better detail. More specifically, the reflections from three rays 102, 104, 106, of illuminating background light are considered. FIG. 2 does not show the liquid crystal material or the transparent cover plate, but only the reflections of the incident light from the whole panel with respect to the positions of reflective electrodes. In fact, the returned rays go through the liquid crystal material, etc.
Ray 102 is received by electrode 54, which is in a reflection mode. (The reflection mode is denoted by showing electrode 54 with hatched lines.) A ray 112 is returned to the viewer. Ray 104 is received by electrode 56, which is in a non-reflection mode. (The non-reflection mode is denoted by showing electrode 56 without hatched lines. It is understood that the external appearance of electrodes 54 in 56 is the same regardless of the mode.) A ray 114 is returned to the viewer. It is to be noted that returned ray 114 is generally much smaller than returned ray 112. Ideally ray 114 has zero magnitude.
The problem arises from the reflection of ray 106. Ray 106 impinges in border space 84, and returns a ray 116 that is of fixed magnitude. Ray 116 arises from the fact that typically the substrate 50 is flat in border space 84. Ray 116 cannot be modulated by being turned on or off, because there is no electrode in the corresponding location for the liquid crystal material. Thus the light from ray 116 is added to the total returned light, which sets a fundamental limit to the amount of optical contrast available from the panel.
A number of approaches have been tried in the prior art. One such approach is to make a width of border space 84 as small is possible. This approach can only be carried so far, as neighboring electrodes can have different voltages, which would result in interference from fringing electric fields by neighboring pixels that are too close.
Another approach, taught in U.S. Pat. No. 5,012,274, is to place an LCD image projection system at the transparent cover plate, so as to diminish the effect of reflections from the border spaces. This approach, however, is expensive because it requires optical components, and also ignores the fact that light reflected from the border spaces will still pass through the optical system, resulting in stray background light in the off condition, thereby limiting the available optical contrast of the device.
The present invention overcomes these problems and limitations of the prior art.
Generally, the present invention provides a reflective liquid crystal display panel that has a two-dimensional array of reflective pixel electrodes. A diffraction grating is provided in the border spaces between the neighboring reflective electrodes. The diffraction grating diffracts the incident light sideways, thereby subtracting it from the total returned light, which increases dramatically the resulting optical contrast of the panel of the invention. As such, the diffraction grating is preferably tuned to the peak wavelength of the ambient illumination.
Preferably the diffraction grating is formed as additional structure on the substrate during fabrication. The preferred diffraction grating uses raised mesa structures arranged in a checker board pattern. The mesa structures define floors between them, and have roofs that are raised from the floors by a quarter wavelength. Thus a whole band of wavelengths is diffracted away.
The foregoing and other features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment, which proceeds with reference to the drawings.